The present invention relates to an A/F (Air/Fuel) ratio detector for use in the measurement or control of the concentration of oxygen in exhaust gas from a burning device such as an internal combustion engine or gas burner.
An oxygen sensor composed of an ion-conductive solid electrolyte (e.g., stabilized zirconia) coated with porous electrode layers (e.g., Pt porous layers) is capable of detecting the concentration of oxygen near a theoretical (stoichiometric) A/F ratio of exhaust gas from an internal combustion engine to thereby detect the combustion efficiency of the engine. Detection is carried out by sensing a change in an electromotive force that is produced by the difference between the partial oxygen pressure of the exhaust gas and that of atmospheric air. This type of oxygen sensor is presently used in numerous applications, for example, in an automobile for the purpose of controlling its internal combustion engine to run at the theoretical air/fuel ratio.
The conventional oxygen sensor exhibits a large amount of change in its output if the operating A/F ratio (which is the weight ratio of air to fuel) is near the theoretical value of 14.7, but otherwise the resulting change in output is negligibly small. Therefore, the output from this sensor cannot be effectively used for an engine operating at an A/F ratio other than near the theoretical value.
Japanese Published Unexamined Patent Application No. 153155/1983 shows an oxygen concentration detector composed of a pair of oxygen-ion-conductive solid electrolyte plates each having an electrode layer on both sides in a selected area close to one end thereof. The two plates are fixed parallel to each other and spaced to provide a gap in an area corresponding to that selected area having the electrode layers. One electrolyte plate with electrode layers is used as an oxygen pump element, and the other plate also having electrode layers is used as an electrochemical cell sensor element that operates in response to the difference in oxygen concentration between the ambient atmosphere and the gap between the two plates. This type of detector has a quick response, but according to experiments conducted by the present inventors, the output of the sensor is ambiguous. That is, when this device is operated in a fuel-rich region having an A/F ratio lower than the theoretical value of 14.7, the direction of change of the output away from the theoretical value is the same as that for operation in the fuel-lean region. Because of the existence of two possible A/F ratios for a single output, the sensor can be used only when it is definitely known whether the burning device to be controlled is operating in the fuel-rich or fuel-lean region. It has been found that it is very difficult to use this detection device for detecting A/F ratio at or near the theoretical ratio, thereby making precise control over the combustion device with a quick response difficult.